There are known various types of liquid crystal devices For example, guest-host-type liquid crystal device has a cell having filled therein a liquid crystal composition which comprises liquid crystal as host and dichroic dye dissolved therein as guest. When a voltage is applied to the cell sufficient to rotate the liquid crystal molecules, the dye molecules rotate along with the liquid crystal molecules and allow changing of light absorption by the cell to thereby effect display. The reflective liquid crystal device employing guest-host mode is excellent in brightness.
Dichroic dyes used in liquid crystal devices are required to have appropriate absorption characteristics, high order parameters, high solubilities for host liquid crystal and excellent durabilities. The order parameter S is defined as “S=(3cos2 θ−1)/2” when molecular long axes of molecules, which receive thermal wobble, are learning a time-average angle lag “θ” from directors. “S” is 0.0 indicating that molecules are in absolute disordered state, on the other, “S” is 1.0 indicating that molecules are in ordered state in which molecular long axes and directors meet in alignment.
There are few known diachronic dyes which can bring about enough high order parameter, and this result in a lowering of the contrast of guest-host-type displays. Some azo and anthraquinone dyes are known as dichroic dyes bringing about relatively high order parameters. For example, some anthraquinone compounds are disclosed in JP-A-62-64886, JP-A-2-178390 and JP-A-10-260386 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, solubilities of such dyes for host liquid crystal, especially fluorine-containing liquid crystal which has often been used in recent years, are so low that liquid crystal displays employing such dyes cannot have sufficient high optical densities.
It is therefore an object of the present invention to provide a liquid crystal composition and a liquid crystal cell which can contribute to improvement of displaying contrast and bring about high optical density, when employed in displaying devices. Another object of the present invention is to provide novel anthraquinone dyes which are useful as dichroic dyes, and in especial novel anthraquinone dyes bringing about high order parameters and having high solubilities for host crystal liquid.